Magnetoresistive elements feature an electrical resistance that strongly depends on the magnitude and/or a direction of an externally applied magnetic field. Magnetoresistive elements therefore in principle provide efficient determination of magnetic field strength and magnetic field direction. For example, when the resistance of an element varies with the angle between the element and the direction of an applied magnetic field, by making use of such an element, a rotation angle can be effectively measured in a touch-less way.
Generally, there exists a large variety of different magnetoresistive elements making use of different fundamental effects. For example, the Anisotropic Magnetoresistive (AMR) effect shows a change in electrical resistance in the presence of a magnetic field. AMR sensors are typically made of soft-magnetic material, such as nickel-iron (Permalloy) thin film deposited on a silicon wafer. The magnetoresistive effect is mainly given by the relative direction between an electrical current and the direction of magnetization.
Another effect denoted as Giant Magneto Resistance (GMR) can be exploited by making use of a stack of alternatingly magnetic and non-magnetic layers. The magnetic layers are typically ferromagnetic. The magnetizations of adjacent ferromagnetic layers are coupled in an anti-parallel way, e.g., in the absence of a magnetic field, and the electrical resistance of such a GMR element strongly depends on the mutual orientation of the magnetization of adjacently positioned magnetic layers. Therefore, conventional GMR elements do not allow a direct determination of a direction of a magnetic field.
However, dedicated GMR systems, denoted as spin-valves also provide determination of a direction of the magnetic field. GMR spin-valve elements feature a magnetic anti-ferromagnetic layer with a fixed spatial orientation, the so called pinning layer. Generally, there is a strong coupling between the pinning layer, a ferromagnetic layer, and the so called pinned layer. A second ferromagnetic layer, the free layer, is adjacently positioned to the pinned layer. The relative orientation of the magnetization between the pinned layer and the free layer determines the electrical resistance of the layer structure. Because the magnetization of the free layer is weakly coupled to the magnetization of the pinned layer, the direction of the magnetization of the free layer follows the direction of an external magnetic field in a parallel way while the direction of the magnetization of the pinned layer cannot follow the direction of the external magnetic field. Hence the spin valve can measure the direction of a magnetic field.
As long as the interaction force between the external magnetic field and the magnetization of the free layer is stronger than the weak coupling force between the free-layer and the pinned layer, the magnetization of the free layer is parallel to the direction of the external magnetic field. As long the interaction force between the external magnetic field and the magnetization of the pinned layer is weaker than the coupling force between the pinned layer and the pinning layer the direction of the magnetization of the pinned layer is independent of the external magnetic field. Hence, by means of a GMR spin valve, the direction of a magnetic field can be determined irrespective of its magnitude, given that the magnitude remains within a predefined margin.
In spin-valve systems as well as with AMR elements the electrical resistance depends on the angle between the magnetization of ferromagnetic layers and an intrinsic direction, the direction of anisotropy. This direction of anisotropy is defined by the pinned layer in GMR-spin valve systems and it is defined by the current in AMR systems. In both cases the direction of an external magnetic field can be determined unambiguously for a range of 0° to 180°.
In principle, determination of a direction of a magnetic field requires an assembly of several AMR or GMR spin-valve elements, that may be arranged e.g., in a bridge circuit, such as a Wheatstone bridge. In such arrangements AMR and GMR spin-valve elements have to be mutually rotated in the plane of sensitivity. Hence, their direction of anisotropy has to point in different directions. This requires manual orientation of selected GMR spin-valves, which is rather disadvantageous in the framework of a mass production process. Alternatively, during a production process, selected GMR spin-valves might become subject to an additional annealing process that serves to rotate the pinning direction of selected elements with respect to the pinning direction of the unselected spin-valve elements. Manual re-orientation as well as performing sophisticated annealing processes feature disadvantages with respect to production costs and production efficiency, especially in mass production processes. For directional magnetic sensors it would be advantageous to make use of an assembly of AMR or GMR spin-valve elements that feature a common pinning direction or a common direction of anisotropy.
Moreover, for some applications like magnetic card readers, magnetic gradiometers or magnetic encoders, a couple of magnetoresistive elements must be positioned and separated by a gap of several tens or hundreds of micrometers or even millimeters. In particular, mass production processes become very complicated when two or more magnetoresistive elements have to be separated in a direction perpendicular to the plane of sensitivity of the planar magnetoresistive elements. Typically, the magnetoresistive elements are manufactured by depositing thin film layers on a substrate. The plane of the layers, e.g., the x-y plane determines the plane in which the magnetoresistive element is sensitive to a magnetic field. Arranging various magnetoresistive elements at a large distance in the z direction requires deposition of a very thick layer of insulating material. Deposition of a layer featuring a thickness of 10 μm up to millimeters by means of sputtering is extremely cost- and time-intensive and therefore undesirable.
It would therefore be desirable to provide a magnetic sensor making use of magnetoresistive layered structures featuring a common pinning direction or direction of anisotropy and providing unequivocal determination of the direction of an applied external magnetic field as well as to provide a magnetic sensor being capable to measure a magnetic field component pointing in a direction in which the magnetoresistive elements are not sensitive to a magnetic field.